In the identification and quantification of impurities in a gas sample, the gas chromatographic process is very useful and popular. In the air separation industries, semiconductor or the so called wafer fab industries, H2 and CO production, CO2 plants and many other processes, the process gas chromatograph (G.C.) is a common and widely used tool to qualify the final product or to control the production process.
A typical chromatographic configuration presently used in the art relies on a simple injection valve, a separation column, a detector, signal amplification and conditioning and, finally, an integration software for peak impurity area calculation and transformation in proper engineering unit.
However, many gas production processes use by-products of another process plant as “raw material input” for a particular gas production. Often, in this type of process, there are many impurities in the raw material.
An example of such process is found in one type of production of high purity H2 from a by-product of another process plant. Hydrogen is a by-product from the production of Sodium Chlorate used for paint production. In this Hydrogen by-product, there are many impurities. Typical impurities are CO2, CO, N2, CL2, H2S, Chloroform, Trichloroethane, Methylene Chloride, Mercaptans, as non limitative examples. In the final H2 product, there could be traces of some of these impurities. In the quality process control, a process gas chromatograph is used to measure impurities in the final H2. Typical impurities measured are O2, N2, Ar, CH4, CO, CO2 and total Hydrocarbons.
Typical chromatograph configurations generally use separation columns made of molecular sieves and various porous polymers. However, with such typical configurations, a problem arises from other impurities that are in the sample and interfere with the impurities to be quantified.
A sample trap could not be installed on the sample inlet line to eliminate the unwanted interfering impurities because such trap will also affect to some extent the impurities to be measured.
In this above-described particular case, the O2 peak is affected by the accumulation of some of the impurities in the separation column. These impurities, which are stopped in the column and define active sites, react with the O2 from the sample. As a result, the O2 peak disappears, leading to false measurements.
Another example of such problem is found when attempting to measure H2 and O2 in a C3 stream, i.e. Propylene 85-95%, Propane≈5-15%, H2 50-500 ppm. In this particular process plant, there is trace of TEAL, i.e. Triethylaluminium, which is a metal alkyl. It reacts violently with air turning it into Aluminium Oxide.
Here again, the O2 peak is affected. The TEAL is stopped by the process G.C. separation column and reacts with the O2 from the sample. The O2 peak decreases slowly to zero after a few injections. Again, in this particular case, a sample trap could not be used on the sample line since it affects some other impurities to be measured.
Another example of such problem is found in some CO production plants. CO at high pressure and temperature react with the Iron from the steel pipe used to carry it. This reaction generates Iron Pentacarbonyl or Fe(CO)5. The Fe(CO)5 also affects the O2 peak in process G.C. The Fe(CO)5 accumulates in the separation column and scavenges the O2 in the sample.
Another adverse effect of Fe(CO)5 on analytical systems was found when attempting to measure hydrocarbons with a FID (Flame Ionisation Detector). The Fe(CO)5 burning in the H2 flame is decomposed and generates Iron Oxide that plugs the FID jet. The FID becomes out of use after only a few days of operation and this with only a few ppm in the sample. CO will also react with Nickel found in some metal gaskets and fittings or filters of the system to generate Nickel Tetracarbonyl, i.e. Ni(CO)4. This metal carbonyl will do the same type of interference as the Fe(CO)5.
Again, in this case, a sample trap cannot be used on the sample line for the same reason cited above.
Also known in the art, there is U.S. Pat. No. 5,612,489 granted to Ragsdale et al. which describes a method to reduce the interference mainly caused by column packing. They suggest the use of a doped carrier gas. They give example with an Oxygen doped Helium carrier gas. So there is at any time some Oxygen amount flowing into the separation column and the detector. They typically dope the carrier gas with less than 10 ppm of O2. This method satisfies the active site that reacts with the impurity to be measured.
However, there is some drawback to this method. First, like any gas chromatograph, when a sample is injected, there is a sudden change in the flow of the carrier gas, and of course in the detector flow. This may result in a strong baseline upset interfering with some impurities to be measured, mainly at low level. In fact, the flow change is changing the dilution ratio when using a dilution scheme to dope the carrier gas. When using a pre-mixed carrier gas, upon injection, the sudden change in column pressure and flow changes the equilibrium of adsorption for O2 or any other reactive gas used to dope the carrier gas, thereby causing a change in the ratio of doping. This also results in baseline upset that may interfere with the impurities peaks of interest. This situation is even more evident at low sample impurity concentration where the need for larger sampling loop is generally required.
Furthermore, for some detectors like the high frequency discharge or plasma emission, the presence of O2 into the discharge zone could quench the ionization resulting in a lower detector response. This leads to some limitation in regards to the quantity of the doping agent. Thus, in case of a strong interfering agent, the above-described prior art method does not perform.
Therefore, it would be desirable to provide an improved chromatographic system and an improved chromatographic method for eliminating the interference problems described above that could be used in standard G.C. configurations. More particularly, it would be desirable to provide a method for eliminating interference from interfering agents, coming from the gas sample itself or from the system material, on the impurities to be quantified in the gas sample.